The invention relates generally to the field of assessing occurrence of chemical and biological pathogens in water, other fluids, particles, concentrated environmental samples, and other milieu.
There are two primary sources of drinking water. The first source, ground water, can be extracted either at springs at which it naturally wells up to the surface or from wells sunk into the earth. Surface water is the second source, and is collected from bodies of stationary or moving water on the surface of the earth, such as rivers, lakes, and reservoirs. Ground water ordinarily accumulates by percolating downward from the surface to underground formations, and is naturally filtered such that it rarely contains particulates carried downwards from the surface. On the other hand, particulates which find their way into surface water can remain suspended therein for significant periods of time.
Some particulates, such as bacteria and protozoa, can affect human health. Such particulates are normally removed or neutralized as a part of the water treatment processes applied to water used for municipal or household purposes. Because some particulate pathogens, such as Cryptosporidium organisms are resistant to most common chemical water disinfection treatments, it is necessary to rely on filtration to remove enough of the organisms to meet the applicable water quality standards.
Protozoa such as Cryptosporidium and Giardia organisms can cause serious illness, particularly in individuals having weakened immune systems. In view of the widespread distribution of municipal water sources, it is of critical importance that protozoan contamination of a municipal water supply be quickly detectable, so that appropriate health warnings can be issued prior to infection of significant numbers of individuals.
Current protozoa detection methods rely on concentration of large volumes of water and detection of protozoa in the concentrated sample using immunological methods (e.g., a fluorescently-labeled antibody which binds specifically to a particular protozoan). The results of the immunological testing must be confirmed by microscopic analysis.
There are numerous shortfalls to immunological detection methods. First, the methods are time-consuming, requiring at least hours to perform. The specificity of the method relies entirely on the specificity of the antibody used. If the antibody reacts with numerous targets other than the protozoan of interest, then a large number of false positive results can be obtained—resulting in unnecessary health alerts, excessive analysis of samples, or both. Potentially more seriously, if the antibody reacts with only certain variants of a protozoan, but not with a variant that occurs in the water being sampled, the immunological test can fail to detect the pathogen even when it is present. Furthermore, current immunological tests cannot differentiate between protozoan cysts (or oocysts) that are infective and those that are not, nor between those which are viable and those that are not. Tests to determine whether protozoa will reproduce or infect subjects can also be performed by observing infection and reproduction of the protozoa in mice or other subjects.
Other methods of indicating the presence of protozoan pathogens in water samples are even less specific. For example, measurements of the turbidity of water samples can provide information regarding the overall content of particulates in the water sample, but cannot identify the particulates. Examination of the presence of indicator organisms (e.g., fecal coliform bacteria) can indicate occurrence of generalized contamination of the water sample, but rely on association of protozoan contamination with fecal contamination.
The methods disclosed in this application overcome the shortcomings of prior art methods and enable detection of protozoan and other particulate contaminants in water samples.
Cryptosporidium 
Cryptosporidia are protozoan parasites that can cause severe, acute disease in humans and other animals when the parasites are ingested. Occurrence of the disease requires reproduction of the parasites in the host. In healthy humans, the parasites can cause severe diarrhea, cramping, and discomfort. Although most healthy humans recover readily from cryptosporidial infection, immunocompromised individuals (e.g., humans who are ill, taking immunosuppressing drugs, very old, or very young) can be much more severely affected. As demonstrated in known outbreaks, cryptosporidial infection can be fatal to immunocompromised patients. There is no specific drug therapy proven to be effective to treat cryptosporidial infections. For these reasons, detection of cryptosporidia in water supplies is important. It is also important to be able to distinguish viable and non-viable cryptosporidia and infectious and non-infectious cryptosporidia.
Environmental sources of cryptosporidia are not exhaustively understood. However, there is a general understanding that at least most cryptosporidia are transmitted by way of fecal contamination, the feces being of either human or animal origin. For this reason, water sources which may at least occasionally be contaminated with treated or untreated sewage or with runoff from agricultural animal farms and ranches are considered to be at significant risk for contamination with cryptosporidia.
Cryptosporidia may be identified by their reaction with specific antibodies and by their microscopic morphological and staining characteristics. Cryptosporidia occur outside the body of an animal primarily in the form of oocysts, which are environmentally stable and resistant particles having a diameter that is typically in the range from about 3-6 micrometers. Each oocyst typically contains four sporozoites, each of which can independently infect a host upon ingestion by the host of the oocyst. Extended exposure to the environment, treatment with certain chemicals, exposure to ultraviolet radiation, and other unknown factors can render sporozoites within an oocyst non-viable (i.e., unable to infect a host upon ingestion of the oocyst).
Microscopic examination of oocysts by a trained expert is a currently known method of differentiating viable and non-viable sporozoites. If an oocyst contains no viable sporozoites, then occurrence of the oocyst in a water supply is not a significant health concern. However, it is difficult to determine by simple microscopic observation whether an oocyst contains any sporozoites, let alone any that are viable. There is currently no practical way of differentiating between oocysts that contain viable sporozoites and those which do not, at least on the scale of municipal water treatment. For this reason, the efficacy of water treatment processes for rendering cryptosporidia sporozoites non-viable can not be practically assessed, and chemical or physical treat water supplies to render the sporozoites non-viable cannot be relied upon to produce potable water. A rapid method of differentiating viable and non-viable cryptosporidial sporozoites could render such treatments practical. The present invention overcomes this difficulty.
Raman Spectroscopy
Raman spectroscopy provides information about the vibrational state of molecules. Many molecules have atomic bonds capable of existing in a number of vibrational states. Such molecules are able to absorb incident radiation that matches a transition between two of its allowed vibrational states and to subsequently emit the radiation. Most often, absorbed radiation is re-radiated at the same wavelength, a process designated Rayleigh or elastic scattering. In some instances, the re-radiated radiation can contain slightly more or slightly less energy than the absorbed radiation (depending on the allowable vibrational states and the initial and final vibrational states of the molecule). The result of the energy difference between the incident and re-radiated radiation is manifested as a shift in the wavelength between the incident and re-radiated radiation, and the degree of difference is designated the Raman shift (RS), measured in units of wavenumber (inverse length). If the incident light is substantially monochromatic (single wavelength) as it is when using a laser source, the scattered light which differs in frequency can be more easily distinguished from the Rayleigh scattered light.
Water exhibits very little Raman scattering, and Raman spectroscopy techniques can be readily performed in aqueous environments. Because Raman spectroscopy is based on irradiation of a sample and detection of scattered radiation, it can be used to analyze water samples with little preparation.
An apparatus for Raman Chemical Imaging (RCI) has been described by Treado in U.S. Pat. No. 6,002,476, and in co-pending U.S. patent application Ser. No. 09/619,371, the entirety of each of which is incorporated herein by reference.